Anisotropic propagation of surface acoustic waves on nitride layers

The anisotropic propagation of surface acoustic modes in GaN and AlN induced by the c-sapphire substrate is presented. In the GaN case, the slow acoustic propagation velocity of GaN compared with sapphire leads to guided modes in the overlayer, which propagate at higher velocities but are more attenuated than the Rayleigh mode. Above the transonic state, pseudo-SAW modes are observed, some of them with low insertion losses. In contrast, only the Rayleigh mode is observed in AlN filters due to its higher acoustic propagation velocity with respect to sapphire. The difference in the crystal structure of the sapphire and the nitrides induces a dependence of the sound velocity of all the modes, and hence their frequency, on the propagation direction. The simulations show very good agreement with the experimental data for both nitride/sapphire structures when the anisotropy induced by the substrate is taken into account.     

Observations of acoustic surface waves in outdoor sound propagation

Acoustic surface waves have been detected propagating outdoors under natural conditions. Two critical experimental conditions were employed to ensure the conclusive detection of these waves. First, acoustic pulses rather than a continuous wave source allowed an examination of the waveform shape and avoided the masking of wave arrivals. Second, a snow cover provided favorable ground impedance conditions for surface waves to exist. The acoustic pulses were generated by blank pistol shots fired 1 m above the snow. The resultant waveforms were measured using a vertical array of six microphones located 60 m away from the source at heights between 0.1 and 4.75 m. A strong, low frequency "tail" following the initial arrival was recorded near the snow surface. This tail, and its exponential decay with height (z) above the surface (approximately e(-alpha z)), are diagnostic features of surface waves. The measured attenuation coefficient alpha was 0.28 m(-1). The identification of the surface wave is confirmed by comparing the measured waveforms with waveforms predicted by the theoretical evaluation of the explicit surface wave pole term using residue theory.     

钽酸镓镧与石英声表面波传播特性对比研究

钽酸镓镧(LGT)是近年研制出的新型压电材料。计算了这种材料在不同切向和传播方向的声表面波传播特性,并和石英晶体上相应切向和传播方向的声表面波传播特性进行了对比分析。计算结果表明这种材料的声表面波传播速度一般比石英低1000 m/s左右,和石英一样,具有零温度切向和纯模方向,但它的机电耦合系数K²却远高于石英。还对目前报道的LGT的不同材料常数进行了对比计算,计算结果表明不同材料常数的声表面波传播特性有明显的差异,其中以频率温度系数差异最大。     

Time delay temperature behaviour of surface acoustic waves propagation on crystal substrate

A method is described for the computation of the time delay temperature behaviour for surface acoustic waves propagated in a surface layer of piezoelectric crystals. The linear field equations for small vibration superposed on thermally induced deformation were employed in the determination of the time delay temperature behaviour. The computed time delay temperature dependence of the surface acoustic waves propagated in the digonal axis direction on the ZX-cut quartz plate is given as an example.     

Quantized charge pumping by surface acoustic waves in ballistic quasi-1D channels

Adiabatic pumping of electrons induced by surface acoustic waves (SAWs) in a ballistic quasi-1D quantum channel is considered using an exactly solvable tight-binding model for non-interacting electrons. The single-electron degrees of freedom, responsible for acoustoelectric current quantization, are related to the transmission resonances. We study the influence of experimentally controllable parameters (SAW power, gate voltage, source-drain bias, amplitude and phase of a secondary SAW beam) on the plateau-like structure of the acoustoelectric current. The results are consistent with existing experimental observations.Received: 24 February 2004, Published online: 12 July 2004PACS: 73.23.-b Electronic transport in mesoscopic systems - 73.50.Rb Acoustoelectric - magnetoacoustic effects - 73.40.Ei Rectification     

The effect of surface stress on the propagation of Lamb waves

This work investigates the possibility of the propagation of Lamb waves in thin solid layers with external traction free surfaces, in the presence of surface elasticity, inertia and residual stress. It is demonstrated that such waves do exist and that their characteristics can be quite different from their classical counterparts. The governing equations with non-classical boundary conditions involving the bulk and surface stress are solved exactly in the frequency-wavenumber domain. This solution is utilized to compute the Lamb wave modes for different layer thicknesses. An efficient strategy to capture all the modes of Lamb waves within a given frequency window is outlined. It is shown that the effect of surface elasticity and inertia becomes significant with increasing frequency and decreasing layer thickness, where the number of modes participating within a given frequency window is more than that permitted by the classical theory. Further, it is observed that the nature of the Lamb wave modes (in terms of negative dispersion) in the presence of surface stress is similar to what predicted by the nonlocal theory and microstructure based continuum theory.     

含有孔隙的层状材料中声表面波传播特性的理论研究

研究含有孔隙的层状材料中Rayleigh波的传播特性。采用本征函数展开法,并利用孔隙率与材料的弹性常数和密度之间的关系,模拟了不同孔隙率情况下铁基氧化铝层状材料中Rayleigh波的相速度色散曲线,分析了孔隙率对铁基氧化铝层状材料中Rayleigh波相速度的影响。Rayleigh波色散曲线的变化规律能同时反映层状材料中弹性模量、泊松比、密度和孔隙率的信息,为含有孔隙的层状材料参数的反演提供了理论依据。     

Modeling of atmospheric propagation of acoustic gravity waves generated by different surface sources

A model of acoustic gravity wave propagation is developed and a numerical algorithm for solving the corresponding hydrodynamics equations is elaborated. Behavior of the waves in the upper atmospheric layers is studied in relation to the parameters of the ground-based sources.     

Imaging of surface acoustic waves

A new experimental method has been devised that directly determines the group velocities of surface acoustic waves. A point source and a point detector are employed to measure the ultrasonic transmission across a solid surface as a continuous function of the propagation direction. Results for single pulses give the times-of-flight for both Rayleigh surface waves (RSW's) and pseudo-surface-waves (PSW's). Calculations and measurements of the group velocities of the surface waves on silicon show some unanticipated behavior: fluid loading qualitiatively changes the group velocity curves for both RSW and PSW. In particular, the RSW branch gains an additional component which we denote here as an induced Rayleigh wave (IRW). If a wave train is employed in the experiment, the analog of phonon focusing is observed for the ultrasonic waves, modified by internal-diffraction effects. Systematic measurements of the wave intensities on silicon as a function of propagation distance are consistent with expected acoustic losses into the surrounding water: the attenuation length of a wave depends on the mode and frequency. A survey of surface-wave images on other crystals is included in this study.     

On propagation characteristics of SH-type surface acoustic waves in periodic metal gratings by variational principle

Based on D.P. Chen and Haus＇ theory, a theoretical method was presented to analyze dispersion characteristics of SH-type surface acoustic waves （SAWs） propagating on periodic metallic grating structures with a variational principle and coupling-of-modes equation. Without using complicated Green＇s function, the calculating results of the method agree well with those of Hashimoto＇s theory. On the other hand, Hashimoto＇s method is helpless for calculating the dispersion relation of short-circuited gratings on ST-90°X quartz etc. However, the method developed in this paper can successfully calculate it.     

Exciton transport by surface acoustic waves

Long-range acoustic transport of excitons in GaAs quantum wells (QWs) is demonstrated. The mobile strain field of a surface acoustic wave creates a dynamic lateral type I modulation of the conduction and valence bands in a double-quantum-well (DQW) structure. This mobile potential modulation transports long-living indirect excitons in the DQW over several hundreds of μm.     

声表面波介质表面受力条件下的波速变化研究

为了计算力负载直接作用在声表面波传播表面时波速的变化特性,通过有效材料系数将负载引入,并用广义Green函数计算声速,得到了数值计算的结果,并用声表面波谐振器进行了实验.实验结果表明,将力负载直接加载在声表面波的传播表面时,波速变化量和力负载大小近似为线性关系,该加载方式下,实验中声表面波谐振器的谐振频率对外加质量负载有较高的灵敏度,可达1900 Hz/g.     

Roughness-trapped shear horizontal surface acoustic waves

Shear horizontal surface acoustic waves do not exist on the flat surface of a semi-infinite elastic medium. It has been shown by several authors recently that such waves can exist on a periodically corrugated, planar surface. We show here on the basis of the Rayleigh method that shear horizontal surface acoustic waves exist on a randomly rough planar surface of an isotropic elastic medium. These waves are only weakly localized to the surface and they have a lifetime that is long due to their roughness-induced scattering into other surface acoustic waves and into bulk waves.     

Pulse propagation of acoustic waves scattered in a channel

When acoustic waves are scattered by random sound-speed fluctuations in a two-dimensional channel the energy is continually transferred between the propagating modes. In the multiple- scattering region the energy flux assumes an asymptotic form in which there is equal energy flux propagating in each mode. Here we shall make use of this well known result to show how to obtain an asymptotic form for a pulse of acoustic energy propagating in the channel. In the multiple-scattering region the speed of the acoustic waves in the pulse continually changes as the energy is transferred between the modes. The process is basically a diffusion process around the mean speed of propagation. We shall first show, using physical arguments, that the diffusion coefficient is proportional to the square root of the propagation distance times the mean free path of scattering. The theory governing the acoustic propagation in the channel is formulated in terms of modal coherence equations and we shall next give a brief review of the definitions of the coherence functions and a discussion of how the equations governing the propagation of the modal coherence functions are derived. We shall then show how the pulse shape and the relevant parameters may be obtained by solving the basic modal coherence equations at large propagation distances.     

On propagation of anti-plane shear waves in piezoelectric plates with surface effect

Based on the surface piezoelectricity model, the anti-plane or horizontally polarized shear (SH) waves propagating in an infinite piezoelectric plate of nano-thickness are investigated to show the surface effect on wave characteristics. The influence on the overall properties of piezoelectric structures resulting from the surface effect is treated as a spring force exerting on the boundary of the bulk. The frequency equations of anti-symmetric and symmetric waves are presented analytically for the electrically short-circuited case. Numerical results show that the wave properties are size-dependent, and the surface effect becomes very pronounced at a high frequency.     

Miniaturization of surface acoustic waves rotary motor

This paper presents the experimental study of a miniaturized surface acoustic waves (SAWs) rotary motor and the theoretical calculation. After the first success in SAW rotary motor operating at 9.85 MHz, a smaller rotary motor is designed. With the operating frequency of 30 MHz and the driving voltage of 120 V(p-p), the motor can rotate at a speed of 270 rpm.